1. Field of the Invention
The present invention relates to a pressure activated electric switch of the so-called “blister” type, in other words where switching is carried out by the deflection, under the effect of an actuator, of a conductive resilient blister dome against two conductors to be connected. More particularly, the invention proposes in this context on the one hand an optimised design and assembly of the parts constituting the switch, making it possible to improve the touch response thereof, and on the other hand a judicious process for manufacturing and packaging both said parts and the assembled switch.
A typical application of the invention is the low voltage switching used in respect of ancillary controls in a motorcar passenger compartment (window winders, seat positioning, sunroof, car radio, on-board computer, unit controls, etc) where the switch is mounted directly onto an electric circuit mounting plate, for example a printed circuit.
2. Description of Related Art
A known example of a blister switch that is particularly advantageous in terms of ergonomics, reliability, and easy installation by automated tools is described in French patent application FR-A-2 800 904 filed in the name of the DAV S.A. company. This blister switch 1, shown in FIGS. 1 to 3, includes a flexible and distortable actuator 2 which covers a switch component in the form of a resilient metal dome forming a blister 4, the latter being positioned just above a switching zone 6 at which are located two terminal points 8 and 10 of respective tracks 12 and 14 of a printed circuit 16. These terminal points may thus be connected electrically via the blister 4 when it is inflected towards the surface of the printed circuit under the action of pressure transmitted by the actuator 2.
The actuator 2 comprises a base 2a at an upper part 2b terminating in an actuation nipple 18 accessible by the user, on which the force required to distort the blister 4 is exerted.
More particularly, the blister 4 is housed in the space 20 (FIG. 2) defined between the upper surface of the printed circuit 16 and a curved internal surface 22 of the actuator 2. The blister 4 rests on the printed surface at several outward curving peripheral support points 24, by which it is kept in a raised position.
The blister has a central incoming part 26 plumb with one 8 of the terminal points, with which it creates the contact, the other terminal point 10 being in permanent contact with the blister in proximity to one of the peripheral support points 24. The base of the actuator is annular and its periphery constitutes a sealing lip 28.
This is housed in a groove 30 provided in sealing means constituted by a hoop 32, which is anchored to the printed circuit 16 by anchoring lugs 34. To this end, orifices 36 are provided in the printed circuit so that the lugs 34 can be clipped on to them. In this way, the printed circuit 16 acts as a support for the hoop 32 and, thereby, for the switching device in its entirety 1. The hoop 32 is made in a material that is rigid relative to that of the actuator 2, and more particularly relative to the sealing lip 28.
As is shown in FIG. 2, the groove 30 is constituted by a clearance comprising a horizontal bearing surface 30a and a vertical bearing surface 30b so as to constitute support zones for the sealing lip 28 and so to improve the seal and the insulation relative to the outside of the switching zone 6. In the non-actuated state, the sealing lip is applied strongly at least to the horizontal bearing surface 30a, which prevents any matter from being introduced into the space 20.
A more comprehensive description of this switch is given in the aforementioned patent document FR-A-2 800 904.
Before assembling the switch on the printed circuit 16, it is necessary to hold the blister 4 in its normal position relative to the actuator 2. To this end, the curved internal surface 22 of the actuator is provided with four pairs of segments or lips 38, as shown in FIG. 3, each pair sandwiching a peripheral edge of the blister 4.
These holding means act permanently on the blister, holding it also when the switch is finally assembled on the printed circuit. In fact, these means are only of use when handling prior to and during the installation of the switch, since the blister is then held correctly by the printed circuit 16, and have the secondary effect of disturbing the touch effect of the blister, and of making the assembly of the switch and the geometry of the actuator 2 more complex.